Comparative quinacrine mustard chromosone banding studies on the karyotypes of the Chinese and Armenian hamsters (Cricetulus griseus and Cricetulus migratorius disclosed 89 percent of the chromosone banding sequences in the two species were homologous. Studies employing reticular cell neoplasms and hyperplastic myeloid Cytomegalovirus-carrier cell lines have revealed: (a) two specific loci on chromosone #1 of the Chinese hamster correspond to sites of breaks and reunions that led to the derivation of two major autosomes (#6 and #7) and the X chromosome of the Armenian hamster; and (b) homologous distal segments of these chromosomes in (a) experience inverted tandem duplications. In contrast to the randomized events leading to the conventional types of rearrangements, the mechanism leading to inverted tandem duplications is expressed en masse, with numerous cells undergoing the identical sequence of events encompassing two cell cycles for its completion. Thus, phylogenetically-related sites in two species express the identical mutational event, thereby demonstrating specific structural (and genic?) redundancies to stem from an ill-defined genetic mechanism. Furthermore, the identical trends were noted among several diploid clonal sublines after being distributed to widely-separated laboratories (San Francisco and Moscow). The fact that the Armenian hamster develops only reticular cell neoplasms may, in part, support the observed association of inverted tandem duplications with chromosome types that are unique for the species, in contrast to other elements of the karyotype which have remained intact in the course of speciation. Comparative studies (karyological, immunological, and mutational) are planned to determine if any overt functional attributes of reticular cell neoplasms may be linked to the presence (or absence) of structural redundancies.